1. Field of the Invention
The invention disclosed herein relates to imaging a subsurface material from a borehole.
2. Description of the Related Art
In exploration for hydrocarbons, it is important to make accurate measurements of various properties of geologic formations. In particular, it is important to determine the various properties with a high degree of accuracy so that drilling resources are used efficiently.
Generally, oil and gas are accessed by drilling boreholes into the subsurface of the earth. The boreholes also provide access for taking measurements of the geologic formations.
Well logging is a technique used to take measurements of the geologic formations from the boreholes. In one embodiment, a “logging instrument” is lowered on the end of a wireline into a borehole. The logging instrument sends data via the wireline to the surface for recording. Output from the logging instrument comes in various forms and may be referred to as a “log.” The log generally includes measurements performed at different depths.
Many types of measurements are made from the borehole to obtain information about the geologic formations. One important measurement is resistivity of the geologic formation. Using resistivity data, an image of the formation can be formed. The resistivity log can be used by geophysicists to determine important characteristics of the formation such as the ability of the formation to contain a reservoir of hydrocarbons.
One type of logging instrument used for measuring resistivity is referred to as a “four-terminal resistivity imager,” FIG. 1 illustrates a four-terminal resistivity imager 99 as is known in the prior art. FIG. 1A illustrates a side view of the imager 99 that is used for imaging a formation layer 97 in a geologic formation 98. FIG. 1B illustrates a front view of the imager 99. Referring to FIG. 1A, the imager 99 includes an upper current injector electrode 91 and a lower current injector electrode 92. The electrodes 91 and 92 are used to conduct electric current (I) 94 through the formation 98 and the formation layer 97. The electric current 94 is supplied by an electric source 95 and measured by an ammeter 96.
Referring to FIG. 1B, the imager 99 includes a plurality of button-sensor pairs 90 where each button-sensor pair 90 includes an upper button-sensor electrode 88 and a lower button-sensor electrode 89. The plurality of button-sensor pairs 90 is disposed between the upper button-sensor electrode 91 and the lower button-sensor electrode 92. Each button-sensor pair 90 is used to measure a formation voltage (Vi) 87.
Referring to FIG. 1A, the imager 99 includes a metal shield 86 used to shield the measurements of formation voltage 87 from the direct electric field created by the voltage between the electrodes 91 and 92.
Referring to FIG. 1B, the distance between the upper edge of the upper current injector electrode 91 and the lower edge of the lower current injector electrode 92 is 14.5 inches. The center-to-center distance between the upper button-sensor 88 and the lower button-sensor 89 is 10 mm.
The response of the imager 99 for the i-th button sensor pair 90 is defined as Vi/I.
The four-terminal resistivity imager 99 of FIG. 1 has a drawback. The drawback is that the imager 99 has little or no azimuthal resolution. “Azimuthal resolution” in this application is related to discerning changes in resistivity in the formation 98 with respect to changes of radial direction from the borehole when the borehole is viewed from above.
Therefore, what are needed are techniques to measure resistivity of a formation from a borehole wherein the resistivity measurements have high azimuthal resolution.